In the manufacture of cellulose esters, the recovery of the organic acid is very important. For example, in the manufacture of cellulose acetate, approximately 4 to 41/2 kilograms (kg) of acetic acid are used per 1 kg of cellulose acetate produced. About 1/2 kg of acetic acid is consumed in the production of 1 kg of cellulose acetate and the remaining 31/2 to 4 kg are discharged from the process. This discharged acetic acid is recovered and recycled into the cellulose acetate manufacturing process.
The principal unit operation in the recovery of the acetic acid, which is in the form of an aqueous weak acid stream comprising approximately between 25-35% by weight of acetic acid, is a liquid--liquid or solvent extraction process. Benzene was formally a component of the extraction liquor. However, concern, regarding the carcinogenic effects of benzene, has caused its use to be curtailed. In its place, new extraction solvents, including ketones, esters, ethers, and alkanes that have a boiling point less than acetic acid, are being evaluated and used when deemed appropriate. With the reformulation of the solvent, the prior processes, which included benzene, have been changed.
One problem that has arisen with the use of these new solvents, particularly the esters (e.g., ethyl acetate and isopropyl acetate), is its hydrolysis to alcohol (e.g., ethyl alcohol and isopropyl alcohol). This hydrolysis occurs principally in the portion of the acid recovery process after the solvent extraction where the solvent and the acetic acid are resolved, typically a distillation process.
This alcohol has a negative impact on the acid recovery process. When the alcohol reaches a concentration of about 2% by weight of the solvent stream, the liquid--liquid equilibrium (LLE) in the extractor is upset. This is manifested by increased water concentration in the extractor overhead stream. Increased water concentration is undesirable for two reasons: 1) the time between cleanings of the solvent/acetic acid distillation system is decreased because more organic salts are carried to it; and 2) the energy demand at the solvent/acetic acid distillation system is increased.
A prior solution to the increased level of alcohol in the solvent stream has been to distill out the alcohol and dispose of it through conventional waste water treatment systems. This solution, however, has two drawbacks: 1) the alcohol is wasted; and 2) the effluent stream carrying the alcohol causes a high chemical oxygen demand (COD) on the waste water treatment facility.
Another consideration limiting options to the solution of the alcohol problem are minor ion forming contaminants in the acid recovery process. These contaminants can form ions that poison ion exchange resins, and thereby limit the use of those resins. For example, acetonitrile will hydrolyze to form acetic acid and ammonium ions. Thus, the use of ion exchange resins in a solution of the problem is suspect.
Accordingly, in view of the above considerations, there is a need to reduce the alcohol level in the solvent stream of these acid recovery processes which: will not impact the liquid--liquid equilibrium of the extractor; will not place unnecessary demands on the waste water treatment facilities associated with the process; and can tolerate the ions generated by the process.